The present invention generally relates to thermal printing heads, and more particularly to a thermal printing head which comprises first through n-th heating elements with respect to one printing line, where n is an integer, and a k-th heating element has an area of M/2.sup.i when a total area of one picture element is represented by M, where i and k are arbitrary natural numbers between one and n.
Recently, it is common to obtain a digital signal by subjecting signals such as a video signal and a graphic signal which is obtained from a personal computer to an analog-to-digital (A/D) conversion, and supply the digital signal to a printing apparatus so as to print a picture or graphics on a printing paper. Printing apparatuses may be roughly divided into impact type printing apparatuses and non-impact type printing apparatuses. An impact type printing apparatus generally comprises a head having a plurality of wire-dots, and characters or the like are printed on the printing paper when the wire-dots strike an ink ribbon placed on top of the printing paper. However, in the impact type printing apparatus, a large mechanical noise is generated during the printing operation due to the striking action of the wire-dots. Further, there is a disadvantage in that the resolution of the printed characters or the like is low.
On the other hand, non-impact type printing apparatus may be roughly divided into ink-jet type printing apparatuses and thermal transfer type printing apparatuses. An ink-jet type printing apparatus generally comprises a head having a plurality of nozzles, and characters or the like are printed on the printing paper when ink is sprayed onto the printing paper from the nozzles. Compared to the impact type printing apparatus, the ink-jet type printing apparatus is advantageous in that only a small mechanical noise is generated during the printing operation. However, there is a disadvantage in that the nozzles are easily blocked by paper and dust particles which adhere to the nozzles.
A thermal transfer type printing apparatus comprises a thermal printing head. Characters or the like are printed on the printing paper when the thermal printing head heats an ink film placed on top of the printing paper, or when the thermal printing head heats the surface of a printing paper which has a special coating. The printing paper which has the special coating darkens at parts where the surface thereof is heated. Compared to the impact type printing apparatus, the thermal transfer type printing apparatus is advantageous in that only a small mechanical noise is generated during the printing operation. Moreover, the problem of paper and dust particles adhering on the nozzles and blocking the nozzles as in the case of the ink-jet type printing apparatus, will not occur in the thermal transfer type printing apparatus. Theoretically, it is possible to use the same thermal printing head when carrying out the printing with respect to the printing paper by use of the ink film and when carrying out the printing with respect to the printing paper which has the special coating.
As one example of a conventional thermal printing head, there is a thermal printing head which comprises a heating dot element having an area which is the same as the area of one picture element. In this conventional thermal printing head, the heating area of the heating dot element is controlled by controlling the voltage or the like which is applied to the heating dot element, so as to print one picture element with a desired tone (gradation). However, it is extremely difficult to linearly control the heating area of the heating dot element. For this reason, it is extremely difficult to print a picture or the like with the desired tone by use of this conventional thermal printing head.
As another example of a conventional thermal printing head, there is a thermal printing head which comprises a plurality of heating dot elements with respect to the area of one picture element. According to this conventional thermal printing head, the heating dot elements are selectively heated so as to print one picture element with the desired tone. When printing one picture element with one tone out of 64 possible tones, for example, one picture element is described by a maximum of an 8.times.8 matrix of dots. In this case, it is necessary to employ a thermal printing head having 8 heating dot elements arranged in a vertical row, and the printing with respect to one picture element is completed when the 8 heating dot elements are selectively heated as the vertical row of 8 heating dot elements move in the horizontal direction by a distance of 8 heating dot elements. Accordingly, in this conventional thermal printing head, it is necessary to provide 8 times the number of heating dot elements required in the aformentioned conventional thermal printing head which controls the heating area of the heating dot element, and the manufacturing cost of the printing apparatus becomes high. Further, since the printing with respect to one picture element is carried out while the thermal printing head moves in the horizontal direction by a distance of 8 heating dot elements, the time it takes to print one picture element is 8 times longer than the time required in the aforementioned conventional thermal printing head which controls the heating area of the heating dot element.